Description of the Related Art
Solid, inherently tacky, elastomeric microspheres are known in the art to be useful in repositionable pressure-sensitive adhesive applications. As used herein, the term "repositionable" refers to the ability to be repeatedly adhered to and removed from a substrate without substantial loss of adhesion capability. Microsphere-based adhesives are thought to perform well in such applications at least in part due to their "self-cleaning" character, wherein substrate contaminants tend to be pushed aside and trapped between the microspheres as the adhesive is applied. Upon removal, the adhesive can then still present a relatively uncontaminated surface for reapplication to the substrate. However, problems with microsphere loss, i.e., microsphere transfer to the substrate, and the resultant need for use of a primer or binder have been recognized in the art.
Numerous references concern the preparation and/or use of inherently tacky, elastomeric acrylate polymeric microspheres which are solid in nature. Such spheres and their use in aerosol adhesive systems having repositionable properties are disclosed in U.S. Pat. No. 3,691,140 (Silver). These microspheres are prepared by aqueous suspension polymerization of alkyl acrylate monomers and ionic comonomers, e.g., sodium methacrylate, in the presence of an emulsifier, preferably an anionic emulsifier. The use of a water-soluble, substantially oil-insoluble ionic comonomer is critical to preventing coagulation or agglomeration of the microspheres.
U.S. Pat. No. 4,166,152 (Baker et al.) describes solid, inherently tacky (meth)acrylate microspheres which are prepared from non-ionic alkyl acrylate or methacrylate monomer(s) in the presence of both an emulsifier and an ionic suspension stabilizer having an interfacial tension sufficient to prevent microsphere agglomeration. Such microspheres are also disclosed in U.S. Pat. Nos. 4,495,318 and 4,598,112 (Howard), where the preparative methods involve the use of a nonionic emulsifier or a cationic emulsifier. All three patents disclose utility as a "reusable adhesive".
U.S. Pat. No. 4,786,696 (Bohnel) describes a suspension polymerization process for preparing solid, inherently tacky (meth)acrylate microspheres which does not require the use of either an ionic comonomer or an ionic suspension stabilizer in order to prevent agglomeration. Rather, the process requires agitation of the vessel charge prior to the initiation of the reaction sufficient to create a suspension of monomer droplets having an average monomer droplet size of between about 5 and about 70 micrometers. In addition to (meth)acrylate monomer, a minor portion of a non-ionic, vinylic comonomer such as, e.g., acrylic acid may be included to modify the "tacky nature" of the microspheres.
U.S. Pat. No. 3,620,988 (Cohen) discloses a method of preparing "bead-type polymers" which involves the use of a water-insoluble polymeric thickening dispersing agent. The method can be applied to produces pressure-sensitive adhesives in the form of coatable bead suspensions, the adhesives comprising a high solids suspension/dispersion of a lightly crosslinked polymer of a higher alkyl acrylate and a tackifier.
U.S. Pat. No. 4,735,837 (Miyasaka et al.) discloses a detachable adhesive sheet having an adhesive layer containing "elastic micro-balls", wherein the microballs partially protrude from the surface of the adhesive layer. The microballs may or may not be tacky. They can be derived from, e.g., (meth)acrylate monomer and an .alpha.-olefinic carboxylic acid monomer via suspension polymerization in an aqueous medium. However, no details as to the nature of the surfactants utilized, etc., are disclosed. The microballs and an adhesive are dispersed in solvent, mixed, and coated, with the ratio of adhesive to microballs being from about 1:10 to about 10:1. This ratio is disclosed to be critical in order that all microballs in the final product, including those protruding from the surface, are completely covered with the adhesive. A range of 1,000 to 150,000 pieces per square centimeter is disclosed as preferred.
DE 3,544,882 Al (Nichiban) describes crosslinked microspheres composed of 90 to 99.5 weight percent of (meth)acrylate ester and 10 to 0.5 weight percent of vinyl type monomer, e.g., acrylic acid, having a reactive functional group through which crosslinking is achieved by reaction with an oil-soluble crosslinking agent. The microspheres are prepared by dispersing in water a solution (in organic solvent) of copolymer prepared by known methods such as solution, bulk, emulsion, or suspension polymerization. (However, the reference notes that in cases where emulsion or suspension polymerization is used with water as a dispersion medium, it is not necessary to make a new aqueous dispersion.) When tacky, the spheres are said to be useful in spray or coated sheet form as "removable adhesive". The stated purpose of the invention is to provide microspheres having a uniform particle size, but it is also stated that the microspheres may contain other monomers such as vinyl acetate, styrene, acrylonitrile, methacrylonitrile, etc., "...to prevent partial transfer of the adhesive when the carrier (backing) is pulled away from the substrate...".
U.S. Pat. Nos. 4,645,783 and 4,656,218 (Kinoshita) disclose a "repeatedly usable and releasable sheet" coated with an aqueous suspension of microspheres obtained by aqueous suspension polymerization (in the presence of a protective colloid comprising casein as a main ingredient) of one or more alkyl(meth)acrylate esters, one or more .alpha.-monoolefin carboxylic acids, and one or more other vinyl monomers. The microspheres are preferably interspersed with finer polymer particles prepared by polymerization of one or more vinyl monomers in an aqueous medium. These fine polymer particles are said to be "... effective in improving the anchorage to the adherend and the adhesion to the substrate after the aqueous suspension prepared in accordance with the present invention is applied to the substrate".
U.S. Pat. No. 3,857,731 (Merrill et al.) and EP 209337 (Smith & McLaurin) both address problems with microsphere adhesive transfer. The former discloses sheets coated with the tacky elastomeric copolymer microspheres of the Silver patent and a binder material which provides sockets in which the microspheres are held by predominately mechanical forces. The latter states that microsphere adhesives could be put to more demanding applications if it were not for the drawback of adhesive transfer. Tacky, elastomeric microspheres are then described which have a composition formed from non-ionic monomers alone or together with a proportion of ionic comonomers. The microspheres further comprise an adhesion promoting monomer having functionality which remains unreacted during polymerization of the monomers and is available for subsequently binding the microspheres through electrostatic interaction or chemical bonding to a substrate or binder-coated substrate. Preferably, the microspheres are derived from at least one alkyl acrylate or methacrylate ester.
In view of the foregoing, it is an object of this invention to reduce or eliminate problems with microsphere adhesive transfer without the need for a separate binder material or for inclusion of an additional adhesion-promoting monomer.
It is a further object of this invention to provide an elastomeric microsphere-based, repositionable pressure-sensitive adhesive which exhibits greater shear adhesion for a given coating weight of adhesive. Thus, the adhesive can support heavier objects.
It is a further object of this invention to provide an elastomeric microsphere-based, repositionable pressure-sensitive adhesive which exhibits greater peel adhesion for a given coating weight of adhesive. This yields a greater amount of tack for an equal weight of microspheres.
It has now been discovered that these objects, and others, which will become apparent from the following discussion may be achieved by preparing microspheres which, in addition to being inherently tacky, elastomeric, infusible, solvent-insoluble, and solvent-dispersible, are also hollow.